I was imagining keeping the Message-ID, while having the first email also include the same value for In-Reply-To — I completely agree that removing the Message-ID would not work out. If the first email always included both fields, thus referencing itself as a reply to itself, I suppose there could be MUAs that would go into infinite loops. So, maybe too risky.

The only effect I can imagine of always setting the In-Reply-To would be to encourage threading where it's not currently happening, so removing the behavior and deleting all these little methods would be very attractive to me if it were my project. :)

It worked!

In T13265#245125, @brhodes wrote:

I merely wanted to have both possible resolutions on the table, one of which was to give the first message the ID later used In-Reply-To.

How old were things before the upgrade? (Roughly: less or more than about a year old?)